How to Calculate Percentages (Quick Guide)
Updated June 2026
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Percentages come up everywhere — tips, discounts, grades, stats. Here are the three calculations you actually need, with plain formulas.
1. A percentage of a number
Result = (Percent ÷ 100) × Number
Example: 20% of 150 = (20 ÷ 100) × 150 = 0.20 × 150 = 30.
2. "X is what percent of Y?"
Percent = (X ÷ Y) × 100
Example: 45 is what percent of 180? (45 ÷ 180) × 100 = 25%.
3. Percentage change (increase or decrease)
Change = ((New − Old) ÷ Old) × 100
Example: a price goes from 80 to 100. ((100 − 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = +25%. A negative result means a decrease.
Quick mental tricks
- 10% — move the decimal one spot left (10% of 240 = 24).
- 5% — half of 10%.
- 1% — move the decimal two spots left.
- Reverse it: x% of y always equals y% of x. 18% of 50 = 50% of 18 = 9.
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Frequently asked questions
How do I add a percentage to a number?
Multiply the number by (1 + percent/100). To add 15% to 200: 200 × 1.15 = 230.
How do I find the original price before a discount?
Divide the sale price by (1 − discount/100). A $60 item after 25% off was $60 ÷ 0.75 = $80.